Community Season 2 Episode 6 (Community s02e06) with title “Epidemiology 206″ aired on Thursday, October 28th 2010 at 08:00 pm via NBC Network.
Here is summary for Community Season 2 Episode 6 :

Pierce and a few other Greendale students ingest a biohazard substance at the school Halloween Party, causing them to exhibit flu-like symptoms and begin turning into zombies. It is up to the rest of the gang to save themselves and the school when Dean Pelton locks them in with the zombie-infected student body.